Ethylene is the building block of the petrochemical industry. Cracking furnaces are the heart of an ethylene plant, producing hydrogen, methane, ethylene, propylene, butadiene, and other cracked gases.
A fundamental issue with thermal cracking of hydrocarbons is coke formation and deposition inside the radiant coils and also inside the inner tubes of the transfer line exchangers along the run. Coke formation results in the need to periodically decoke a furnace. Decoke is required when radiant coil inlet venturi pressure ratio (“VPR”) reaches the end of run limit (“EOR”) for uniform flow distribution through the coils or maximum tube metal temperature (“MTMT”) of the radiant coils at EOR is reached in any one of the radiant coils.
Decoke air together with dilution steam or medium pressure steam is often used during the decoking process to remove coke deposited in the radiant coils. During furnace decoke, two types of decoking process are traditionally applied; either decoke effluent from the last transfer line exchanger is directly routed to the bottom of the firebox for combustion of coke particles with combustion air, or decoke effluent from the last transfer line exchanger is directly routed to the decoke cyclone for separating coke particles from decoke air and steam. After coke removal, decoke effluent goes to atmosphere via a vent stack. Coke is collected at the bottom of the decoke cyclone.
However, emission requirements have become extremely stringent to comply with Environmental Protection Agency (“EPA”) and National Ambient Air Quality Standard (“NAAQS”) requirements. An ethylene plant cannot be built if the emission requirements are not satisfied.
The size of emitted particles is directly linked to their potential for causing health problems. Particles less than 10 microns (“PM10”) pose the greatest problem for health as they can get deep into the lungs and reach the bloodstream.
Ethylene cracking furnaces also produce flue gas containing pollutants such as nitrogen oxides (NO and NO2), carbon monoxide (CO) and particulate matter. The increasingly stringent environmental regulations in the United States and elsewhere require new control methods to minimize these atmospheric pollutants, including particulate matter with a diameter of 2.5 μm or less (“PM2.5”). For example, recent requirements for projects along the United States gulf coast have required not more than 0.01 Lb/MM Btu (HHV basis) or 10 PPMV (dry basis) of NOx, 0.0025 Lb/MMBtu (HHV) of total PM10+PM2.5, and 0.012 Lb/MMBtu (HHV) of carbon monoxide. Such controls are needed during normal cracking mode and also during steam plus air decoke mode.
Based on current ultra-low NOx burner technology, it is impossible to achieve the Lowest Achievable Emission Rate standard of 0.01 Lb/MMbtu (HHV) of NOx in an ethylene cracking furnace stack. To achieve this standard, a selective catalytic reduction (“SCR”) unit integrated with an ammonia injection grid (“AIG”) must be installed in the convection section of the furnace.
Another furnace emission is the decoke effluent. Conventional decoking passes the effluent through a decoke separator then vents it to atmosphere. This approach probably cannot meet the standards for CO removal and also has limited ability to remove PM2.5.
An alternative process involving routing decoke effluent to the firebox also cannot achieve the requirements for complete particle removal. This is particularly true during furnace decoke. Further, using such a process in cracking furnaces integrated with an SCR unit in the convection section may foul the SCR catalyst.
Accordingly, it is desirable to provide a decoke unit for ethylene cracking furnaces that meet new and anticipated standards for the removal of nitrogen oxides, carbon monoxide, and particulate matter. It is further desirable that such a decoke unit be compatible with existing technology such as SCR units.